There are basically two methods of drilling now in use.
The first method, termed rotary drilling, is illustrated by FIG. 1. A drilling tool or bit TR driven by a tubular shaft or drill pipe OR equipped, from the bit upwards, with first and second stabilizers SA and SB. The shaft is rotatively driven from ground level. Drilling mud descends through the shaft to the bit and goes back up around the shaft.
The second method, termed downhole motor drilling, is illustrated by FIG. 2. The bit TM is driven by a shaft OM, which is itself driven by a downhole motor MF--usually a turbine--and operated by the drilling mud descending under pressure through an axial channel in a drill stem GM. The shaft is held in relation to the stem by means of a certain number of friction bearing disks DA. The drilling mud penetrates into an axial channel CA of the shaft below the disks and motor through special openings LM.
Regardless of the drilling method used, the values of real stresses exerted on the drilling tool down in the hole (the torque, the weight and the lateral stresses on the tool) are only very approximately known on the rig floor in the absence of downhole measurements. Indeed, the torque applied to the drill stem at the surface by the rotary table arrives at the tool diminished by all the losses due to the friction arising between the rods and stabilizers and the walls of the well, all of which losses are highly variable and difficult to evaluate. For the same reasons, knowledge of the total weight on the tool affords a roughly worthwhile indication of the actual axial thrust received by the tool only for near-vertical holes, and the lateral stress on the tool as well as the rod bending stresses are a complete mystery.
Given the conditions surrounding operation of the downhole parts and their shapes and dimensions, it is moreover difficult to envisage placing strain gauges on such shafts, either by bonding or deposition as taught by the latest techniques. Yet such strain gauges are known to be well suited to the measurement of the stresses under consideration.